Weathering steel, also named atmospheric corrosion-resistant steel, is described in Chinese Patent CN1609257 titled “High Strength Weathering Steel With Acicular Structure And Process Of Producing Same”, Chinese Patent CN1986864 titled “High Strength, Low Alloy, Atmospheric Corrosion-resistant Steel And Process Of Producing Same”, Japanese Patent JP04235250A titled “High Corrosion Resistant Steel Sheet” and U.S. Pat. No. 6,315,946 titled “Ultra Low Carbon Bainitic Weathering Steel”, etc. The steel in each of the above patents belongs to the type of traditional CORTEN steel family, and their compositional system belongs to Cu—P—Cr—Ni family or Cr—Mn—Cu family. Different structural morphologies are obtained under certain rolling process conditions by addition of other trace alloy elements, so as to achieve the desired mechanical properties and corrosion resistance. In terms of alloy composition, they are low Cr weathering steel having a Cr content of 0.7% or less and an Al content of no more than 0.1%.
However, the reliance on P, Re to improve corrosion resistance has the problems of phosphorus segregation, cracking, and difficulty in controlling the Re content. In order to further improve the atmospheric corrosion resistance, researchers turn to development of high-alloy type weathering steel by increasing the contents of other corrosion-resistant elements to a large degree. Examples include Japanese Patent JP01079346A titled “Marine Corrosion Resistant Steel”, Japanese Patent JP05302148A titled “Highly Corrosion-resistant, Highly Magnetic Damping Alloy”, Japanese Patent JP10025550A titled “Corrosion Resistant Steel”, Japanese Patent JP2000336463 titled “Corrosion Resistant Steel In The Soil”, and Japanese Patent JP2002285298 titled “Cr-containing Corrosion Resistant Steel For Building And Construction Structure”.
The steel in each of the above five patents contains a relatively large amount of Al, Cr components, and achieves particular mechanical properties with the assistance of other alloy elements. The first two patents relate to high Al weathering steel, wherein the Al content in the steel of Patent JP01079346A is up to 7-20%, while the steel of Patent JP05302148A comprises Si, Cr at levels far higher than common weathering steel in addition to a high Al content. The other three patents relate to weathering steel of high Cr family in terms of compositional system, wherein the content of Cr is generally 7% or higher, mostly in the range of 9-14%. The steel of Patent JP10025550A even contains up to 0.45-0.65% C. In addition, the steel of the above patents also comprises various amounts of components Co, W, Mo, B, Zr, etc. High Al and Cr family weathering steel represented by the above patents comprises such high amounts of alloy components that steel-making and steel-rolling become more difficult in the production on the one hand, and the cost is increased greatly on the other.
The relative corrosion rate of the prior art weathering steel is generally not high when good mechanical properties are guaranteed. Furthermore, the comprehensive mechanical properties of some types of steel can not even be guaranteed; instead, only one mechanical property is superior. As such, the requirements of increased corrosion resistance of steel used for railway vehicles and the like can not be satisfied, leading to short service life and high maintenance cost.